1. Field of the Invention
This invention relates in general to a system for tensioning a string of casing extending between a subsea wellhead and a surface wellhead located on a jackup drilling rig, and in particular to a system utilizing an adjustable mandrel.
2. Description of the Prior Art
Offshore wells may be drilled by floating drilling vessels, or if the water isn't too deep, by jackup drilling rigs. In a jackup drilling rig, the legs of the rig are adjustable in length to support the rig on the ocean floor. The well will have a subsea wellhead assembly located on the sea floor.
In a common subsea wellhead assembly, called a mudline suspension system, the rig will drill to a first depth and install large diameter conductor pipe. The conductor pipe extends to the rig where a surface wellhead will be installed. The surface wellhead is located on a well deck generally about 90 feet below the rig floor but above the water. The operator will then drill to a second depth and install a string of outer casing. The operator drills to a third depth, installs intermediate casing, and then to a fourth depth, installing inner casing. In some cases, an even smaller diameter string of casing may be installed within the inner casing.
In a mudline suspension system, each string of casing has a lower section which extends from the subsea wellhead downward into the well and an upper section which extends from the subsea wellhead to the drilling rig. Each string has a casing hanger located at the upper end of the lower casing string which lands in the previously installed casing string. Each string has lower running tool which connects the lower end of the upper casing string to the hanger.
In a mudline hanger system, seals are not employed between the various strings of casing at the subsea wellhead. Rather, sealing between the various strings of casing is handled at the surface wellhead. When running the various strings of casing, the lower casing strings will be cemented in place. Each casing hanger and its running tool have a number of wash ports. The wash ports are used to circulate fluid down the upper string of casing, through the wash ports at the hanger level, then back up the annulus surrounding the upper casing string to wash the annulus at the casing hanger free of any cement. The wash ports of the intermediate and inner hangers are opened by partially unscrewing the running tool from the hanger after the lower casing string has been cemented. This requires opposite direction rotation, which is normally left-hand rotation. The mating threads between the running tools and their hangers readily allow left-hand rotation without unscrewing any of the sections of the upper string of casing.
It is desirable to support the upper strings of casing between the subsea and surface wellheads in tension. This may be accomplished with a locking member at the surface wellhead. There are various types of locking members, but each is axially moveable relative to the casing and lands on an internal shoulder in the surface wellhead. Tension is applied to the upper strings of casing during each installation, and the locking member is positioned to hold the tension in the upper casing string. Prior art types of locking members include both threaded rings which are rotated into position, and ratchet-type rings which are moved axially without rotation.
In the prior art tensioning techniques, the operator pulls tension with the drilling rig elevators, then moves the locking member into place on the load shoulder. The locking member will be spaced above the load shoulder during cementing and washing out, and therefore clearances exist to allow circulation up the annulus past the locking member. Previously, the locking member could not be landed on the internal shoulder and held in position while applying tension, because the engagement of the lock ring with the internal shoulder blocks flowby for cementing and washout.
In one prior art type which employs a ratcheting locking member, an upper running tool secures to a mandrel-type casing hanger which has exterior grooves engaged by the locking member. In this type, after the locking member has been ratcheted into place on the internal shoulder, the upper running tool is rotated to the left to unscrew it from the mandrel for subsequent operations. It is important to prevent the mandrel and upper running tool from unscrewing while the lower running tool is being unscrewed to expose the wash ports. To prevent inadvertent releasing of the upper running tool while opening the wash ports, in one prior art type, torque fingers are mounted to the upper running tool. Each torque finger has a lower end that engages a torque slot formed in the mandrel. The lower ends are radially expandable, allowing them to spring out of engagement with the slots when the operator later wishes to disengage the upper running tool from the mandrel.
In this prior art type, the ratchet ring was moved downward onto the internal shoulder after cementing and washout operations were completed. Prior to installing the ratchet ring on the internal shoulder, the ratchet ring was held in an upper position surrounding the torque fingers. This upper position of the ratchet ring prevented the torque fingers from moving to the outer position until the washout had been completed. It was not possible to land the ratchet ring on the internal shoulder before cementing, because this would result in the torque fingers releasing from the upper running tool before the washout operation occurred. Because of this reason and the need for circulation flowby discussed above, the prior art ratchet-type mandrel hangers required landing of the ratchet ring after cementing.